stockpiles: implement GUI controls
stockpiles: integrate gui plugin into dfhack.init-example
update NEWS and README with new stockpiles UI info
stockpiles: fix usage help
Create stocksettings directory on startup if necessary
stockpiles: more error handling & cleanup

* prevent crashes when the path doesn't exist
* remove duplicated functions
* sort file list case insensitively
stockpiles: user friendly error handling

* display errors in the GUI when necessary
* handle older versions of DFHack without the persist module
stockpiles: fix loading crash
fix build error on windows

with certain #include permutations the lack of this define would
break the build on windows as windows.h pollutes the namespace with min
and max macros that conflict with std::min and std::max.